PART 2: Common-Emitter Amplifier
INTRODUCTION
In this lab, the student will be working with Common-Emitter Amplifiers. The goals of this lab are to become familiar with gain/bandwidth, CE amplifier, and LTspice. The student will also learn how to use LTspice to analyze the CE amplifier circuit. To conclude the lab, the student will also learn to breadboard and test a CE amplifier circuit. The LTspice exercises were completed and submitted for the prelab.
2.4 Build and Test a Common Emitter Amplifier
To begin, the student constructed a circuit similar to Figure 2.8 from the lab manual, shown below in Figure 2.1. The DMM was then used to probe voltage at the transistor’s base, emitter and collector shown in Table 2.1.
FIGURE 2.1: Breadboarded CE Amp and DC Circuit
Then, the student added the AC circuit to the breadboard, similar to Figure 2.11, with a 1 kHz signal and a 10mV amplitude from the function generator(Figure 2.2). Using the dual capability of the oscilloscope, the student viewed the input and output signals of the circuit. The amplitude voltage gain was determined for the 1kΩ load resistor by taking the ratio of output voltage amplitude to input voltage amplitude. The gain was then tested to see how it is influenced by the load resistance. A series of resistors were measured and recorded in Table 2.2. The data was used to plot gain vs resistance in Figure 2.14. Considering that the audio amplifier will be driving an 8Ω impedance speaker, the CE amp should perform driving this particular load.
Figure 2.2: CE Amp with AC Circuit Added
For the last part of the lab, the student replaced the load resistance back to 1kΩ and measured the gain at different frequencies. As the frequency was changed, the time base on the oscilloscope also needed to be adjusted. The input amplitude also required fine adjustment to maintain a constant value for use in calculating gain. Table 2.3 shows the data that was obtained, along with the gain in dB. Using this data, the Gain (dB) was plotted against frequency data to create a Bode Plot in Figure 2.15. The estimated 3 dB bandwidth, or halfpower bandwidth, for the plot is around 33dB.
Bode Plot
CONCLUSION
In this lab, there were some difficulties placing the capacitors the correct way in the circuit. This mistake felt like a good lesson, to be more careful when building circuits and to pay more attention to the direction the capacitor needs to be placed. This lab was also great practice to prepare for the other labs for the class this semester. A lot of knowledge was gained about CE amplifiers, building useful skills for the future.